1. Field of the Invention
The invention relates to a spark plug for an internal combustion engine improved to positively effect a self-cleaning action.
2. Description of Prior Art
In a spark plug used for a high compression engine and lean burn enging, an ignitable condition deteriorates to cause smolder on a front end surface of the insulator by depositing unburned material thereon such as carbon, oily residue or unburnable fuel ingredient.
For this reason, it has been demanded to burn out the unburned material deposited on a front end surface of the insulator. In order to effectively burn out the unburned material to improve a carbon-fouling resistance, an auxiliary gap type spark plug is disclosed by Japanese Patent Publication No. 58-40831 (referred to as "a first reference"hereinafter) in which an annular space is formed between a front end of an insulator and a diameter-reduced portion of a center electrode so as to define an auxiliary spark gap between a front end surface of the insulator and an inner side of an outer electrode.
In an auxiliary gap type spark plug disclosed by a Provisionally Published Japanese Patent Application No. 2-181383 (referred to as "a second reference" hereinafter), a carbon deposit piled on the insulator is burned out by spark discharges induced from a high voltage power source. Namely, it is possible to remove the carbon deposit exposed to an ionized region due to the induced voltage among the spark discharges across the center and ground electrodes.
In U.S. Pat. Nos. 4,845,400, 5,124,612 and 5,159,232 (referred in turn to as "a third, fourth and fifth reference"hereinafter), it is possible to burn out the carbon deposit piled on the insulator and exposed to the induced voltage in the same manner as described in the second reference.
In the auxiliary gap type spark plug disclosed by the first reference, however, it is not satisfactory to effect the self-cleaning action for the shortage of technology which enables to positively creep the spark discharge along the front end surface of the insulator.
In the auxiliary gap type spark plug disclosed by the second reference, it is necessary to determine a distance between a front end of an insulator and that of a center electrode to be 1.1 mm or less in order to located the front end of the insulator within the ionized region. Namely, the distance is such that the front end of the insulator positions within the ionized region even in the case in which a spark-initiated point with the center electrode is not in the front end of the insulator.
In the spark plug disclosed by the third reference, it is necessary to determine a front end dimension of a center electrode to be 1.0 mm or less from a front end of an insulator upon considering a diametrical relationship between the front end of the center electrode and an axial bore of the insulator.
In the spark plug disclosed by the fourth reference, it is necessary to provide a stepped portion with a center electrode which is placed within an axial bore of an insulator.
In the spark plug disclosed by the fifth reference, it is necessary to establish a dimensional relationship as 0.6 mm.ltoreq.d.ltoreq.1.55 mm in which (d) shows a diameter of a diameter-reduced front end of a center electrode. This is because of the necessity of providing an annular space between an inner wall of an axial bore of an insulator and the diameter-reduced front end of a center electrode.
Therefore, it is a main object of the invention to provide a spark plug which is capable of positively effect the self-cleaning action when the carbon-related deposit is piled on the front end surface of the insulator.